A variety of devices (circuits) are mounted in a semiconductor device depending on its functions. As such devices (circuits), central processing units (CPUs) which process data can be given, for example. Low power consumption is required for CPUs.
In addition, a memory device which is used to process data is mounted on a CPU. As an example of such a memory device, a latch memory can be given.
On the other hand, in recent years, metal oxides having semiconductor characteristics (hereinafter, referred to as oxide semiconductors) have attracted attention. Oxide semiconductors can be applied to transistors (see Patent Documents 1 and 2).